Traditional multicast distribution of Internet Protocol (IP) packets are supported via IP multicast routing and forwarding, which use protocols such as Protocol Independent Multicast (PIM) or Multicast Label Distribution Protocol (LDP) to create multicast replication states on the nodes along the multicast distribution tree in the network. Packets flowing through the network will be replicated to the proper set of neighbors following the replication states on each router.
The multicast forwarding states are hard to aggregate since each application may have a different set of participants, therefore the multicast distribution trees for the applications can all be different. This can cause an explosion of multicast states in the core of the network where all the multicast traffic are passing through.
One prior art technique of reducing the amount of multicast state stored at routers is to encode the multicast membership information directly in the packet header. Bit Index Explicit Replication (BIER) is an example of such a technique.
BIER forwards multicast packets without requiring any intermediate nodes to maintain per-flow state. When a multicast data packet enters a domain, an ingress router determines the set of egress routers to which the packet needs to be sent. The ingress router then encapsulates the packet in a BIER header, which contains a bitstring in which each bit represents exactly one egress router in the domain. Bit Index Explicit Replication (BIER) is defined in IETF Multicast using Bit Index Explicit Replication (published Oct. 16, 2014 as draft-wijnands-bier-architecture-01).
When the multicast group is densely populated (many members are receiving packets for the group), the BIER encoding is efficient. However, in reality, most of the multicast groups are relatively sparse, when compared to the total number of routers in a network. Usually, only a small subset of the routers in the network are participating in any given multicast group (with the rare exception of few popular groups that draw large audiences). In these cases, the bitstring encoding used in BIER would end up having very few one (1) bits and many zero bits, which results in inefficient encoding.
In the case when group membership spans across multiple regions, ingress replication will be used, creating even more inefficiencies due to unnecessary packet replication at ingress.